Carbon fibers are used in a wide variety of fields as carbon fiber-reinforced composite materials that are formed by being compositized with a resin (hereinafter, described as a matrix resin) such as an epoxy resin, an unsaturated polyester resin, a vinyl ester resin or an acrylic resin, and then molded.
Regarding the method for producing a carbon fiber-reinforced composition, a method of impregnating fibers as a reinforcing material with a matrix resin is generally used. Examples of the method of impregnating fibers with a matrix resin include a prepreg method of thinly applying a matrix resin on a releasable paper and spreading fibers thereon in one direction; and a dipping method of passing fibers through a matrix resin bath.
Furthermore, regarding the molding method, there are known a method of laminating sheet-like articles and thermally curing the laminate under pressure using an autoclave; pultrusion molding of mixing one to several hundred carbon fiber bundles, impregnating the carbon fiber bundles with a matrix resin, and then curing the matrix resin through a die, a mold or the like; and a hand lay-up method of impregnating a textile base material such as a fabric or a sheet-like article with a resin at normal temperature, and directly curing the resin.
A carbon fiber-reinforced composite material formed from carbon fibers and a matrix resin by using the carbon fibers as a reinforcing material, is lightweight and has excellent strength and elastic modulus. In regard to such a composite material, development of applications thereof is underway in a wide variety of fields as a material for constituent components for sports and leisure goods, a base material for vehicles and aerospace crafts, and an industrial material for energy and civil construction. Therefore, there is a very strong demand for an enhancement of the performance of carbon fibers as a reinforcing material.
Particularly, for the carbon fibers that are applied as a structural material for vehicle and aerospace applications or as an industrial material, development intended for an increase in strength and an increase in elastic modulus is underway. Such a composite material for the applications as a structural material or an industrial material needs to have a high level of tensile strength in the longitudinal direction of the fiber. However, since orientation disorder or tortuosity of the carbon fiber filaments occurs in the pultrusion molding and hand lay-up method described above, there has been a problem that it is difficult for the mechanical properties such as tensile strength expected from a carbon fiber-reinforced composite material to be exhibited.
Furthermore, a carbon fiber in general is a filament having a diameter of about 5 μm to 8 μm, and is used in the form of several thousand to several ten thousand units of this single fiber being collected together (hereinafter, described as “carbon fiber bundle”). Since carbon fibers themselves have low elongation and exhibit brittleness, carbon fibers are prone to fuzzing due to mechanical friction and the like, and fuzzing and yarn breakage are prone to occur during the production process for a composite material. Therefore, for the purpose of suppressing the occurrence of fuzzing, carbon fibers are often subjected to a sizing treatment by applying various sizing agents. Furthermore, carbon fibers are generally used in the form of a fabric or the like produced by processing the carbon fiber bundles using a weaving machine. In order to produce a high quality carbon fiber-reinforced composite material in an industrially stable manner, it is required that in the process of impregnating fibers with a matrix resin, impregnation of carbon fiber bundles with the matrix resin be achieved readily and completely. However, carbon fibers in their original state lack wettability against matrix resins, and are not easily impregnated with a matrix resin. Therefore, it is difficult to obtain a fiber-reinforced composite material having a sufficiently satisfactory product quality. Even for the purpose of improving this, it is effective to perform a sizing treatment on carbon fibers.
That is, carbon fibers are subjected to a treatment using a sizing agent, for the purpose of enhancing product quality by enhancing handle ability of the carbon fibers, further enhancing wettability against a matrix resin, and thus manifesting the mechanical properties such as tensile strength expected from a carbon fiber-reinforced composite material at a high level.
Patent Document 1 proposes a sizing agent that uses polyglycidyl ethers and the like (hereinafter, referred to as “sizing agent 1”), and Patent Document 2 and Patent Document 3 each propose a sizing agent containing, as essential components, an epoxy resin, a condensate between an unsaturated dibasic acid and an alkylene oxide adduct of a bisphenol compound, and an alkylene oxide adduct of a phenol compound selected from monocyclic phenols and polycyclic phenols (hereinafter, referred to as “sizing agent 2”).
The sizing agent 1 has excellent impregnating ability or interfacial adhesive force; however, it cannot be said that the sizing agent 1 has satisfactory adhesiveness to radical polymerization type resins such as unsaturated polyester resins, vinyl ester resins, and acrylic resins.
Furthermore, the sizing agent 2 can be expected to have enhanced adhesiveness to matrix resins, particularly unsaturated polyester resins, and when an epoxy resin is used as a matrix resin, the sizing agent 2 enables the properties of the fiber-reinforced composite material to be maintained. However, it cannot be said that the sizing agent 2 has satisfactory adhesiveness to radical polymerization type resins.
Patent Document 4 proposes a sizing agent including an ester resin containing one or more epoxy groups, urethane acrylate, an anionic emulsifier and a small amount of a nonionic emulsifier (hereinafter, referred to as “sizing agent 3”).
The sizing agent 3 has excellent adhesiveness to radical polymerization type resins such as unsaturated polyester resins, vinyl ester resins and acrylic resins, and can provide performance that is equivalent to that of composite materials containing epoxy resins as matrix resins. Furthermore, the sizing agent may also have satisfactory suitability to epoxy resins, and can exhibit excellent mechanical strength in composite materials combined with a wide range of thermosetting resins.
However, this sizing agent cannot be said to have a satisfactory effect of suppressing orientation disorder or tortuosity of carbon fiber filaments at the time of molding processing, which causes a decrease in strength of a carbon fiber-reinforced composite material.
Furthermore, Patent Document 5 and patent Document 6 each propose a sizing agent containing a polyurethane resin (hereinafter, referred to as “sizing agent 4”). When a sizing agent contains a polymer compound such as a polyurethane resin, it is effective in suppressing the phenomenon of orientation disorder or tortuosity of carbon fiber filaments at the time of molding processing. However, since the sizing agent 4 substantially uses a polyurethane resin at a proportion of 100%, and is designed for thermoplastic resin reinforcement such that the softening temperature of a dried coating film of the sizing agent would be 50° C. to 150° C., in the case of performing a resin impregnation operation near room temperature as in the case of a composite material using a radical polymerization type resin, resin impregnating ability is poor.
As such, a sizing agent that is effective against defective resin impregnation, or against the orientation disorder or tortuosity of carbon fiber filaments at the time of molding processing, which are causative of a decrease in strength of a carbon fiber-reinforced composite material, has not been found heretofore.